I am of the impression from past articles that Wal-Mart uses Murphy Oil Corporation out of El Dorado, Arkansas as much as possible. In areas where that is not possible, they buy independently from an area low cost source. Such as Tesoro Corp and Sunoco.
I'm waiting for the prices to drop again but it looks like OPEC is going to do something about it once again. We may have seen the lowest prices that we will ever see again unless things get very very bad.
Since 2007 ALL gas sold in the US has ethanol up to 10% as they outlawed MTBE and ethanol is the replacement. Unless of course the referenced station is home brewing their own.
BTW- Price of gas around here has increased ~40 cents a gallon in 2 weeks. But cost of crude has also risen to $46 and hit a low of $36 two weeks ago. I just filled up yesterday afternoon at Hess and paid $1.86 but later in the day they raised the price to $1.92.